Stress relief layer providing high thermal conduction for a semiconductor device

ABSTRACT

A semiconductor chip (27) is soldered on an electrode plate (24) with a thermal relaxation plate (40) therebetween. The thermal relaxation plate has a frame member (41) made of covar or invar and a plate member (42) made of copper. The plate member is inserted into the window space defined in the frame member and is united with the frame member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a solderingmethod for its manufacture, and particularly to improvement of a thermalstress relaxation member equipped to relax the thermal stress in asemiconductor chip and a method of soldering the semiconductor chip on aprescribed member using the improved thermal stress relaxation member.

2. Description of the Background Art

As known well in semiconductor device manufacturing technology, thereare many cases wherein a semiconductor chip formed with desired activeregions is soldered on a prescribed member such as an electrode plate.FIG. 6 shows a partial section showing conventional typical solderedstructure of a semiconductor chip 9. As shown in FIG. 6, a heatdiffusion plate 1, an insulating base plate 3, an electrode plate 5 anda thermal stress relaxation plate 7 are soldered consecutively by usingsolder layers 2, 4 and 6. The semiconductor chip 9 is soldered on thethermal stress relaxation plate 7 through a solder layer 8. In case thesemiconductor chip 9 is a power transistor, for example, the electrodeplate 5 corresponds to a collector electrode plate. In addition, theother electrode plates for the semiconductor chip 9, that is, an emitterelectrode plate and a base electode plate (not shown) are connected tothe semiconductor chip 9 through metallic wires 10.

Among these members, the thermal stress relaxation plate 7 is equippedin order to relax thermal stress generating within the semiconductorchip 9 during soldering process. In case the semiconductor chip 9 ismade of silicon and the metallic collector plate 5 is made of copper,since coefficient of thermal expansion of the collector metallic plate 5is around five times as much as that of the semiconductor chip 9,difference between respective shrinking is great when these are cooledfrom high temperature conditions in soldering. Accordingly, if thethermal stress relaxation plate 7 is not equipped, the thermal stresscorresponding to this difference is directly applied to thesemiconductor chip 9 to deteriorate electric characters and strengththereof. Furthermore, the thermal stress relaxation plate 7 is effectivefor relaxing thermal stress due to heat which is generated duringelectric current flow through the semiconductor chip 9.

Since the thermal stress relaxation plate 7 is provided for thesereasons, the thermal stress relaxation plate 7 must be made of materialof which coefficient of thermal expansion is comparatively similar tothat of silicon. Besides since heat which generates during operation ofthe semiconductor chip 9 must be quickly transmitted to the heatdiffusion plate 1, large thermal conductivity is necessary for the plate7. Therefore, a molybdenum flat plate is widely used as a conventionalthermal stress relaxation plate 7.

However, the molybdenum flat plate is formed by sintering and is veryhigh in the cost, so that the cost of the semiconductor device equippedwith the structure in FIG. 6 is also high.

In addition, as thermal conductivity of the molybdenum is not so large,and efficiency of transmitting to the heat diffusion plate 1 the heatgenerating during operation of the semiconductor chip 9 is not so high.In other words, in case the molybdenum flat plate is used as the thermalstress relaxation plate 7, it is difficult to reduce transient thermalresistance thereof. This problem turns out to be especially serious upona transistor for large electric power in which high speed switchingoperation is repeated.

In order to cope with such a problem, technology using a copper plateembedded with carbon fibers is disclosed in Japanese Patent Laying-OpenGazettes No. 58-32423 (1983) and No. 57-124459 (1982). In thistechnology, high thermal conductivity is secured by copper andcoefficient of thermal expansion is lowered by carbon fibers.

However, in order to suppress thermal expansion of the copper plate, alot of carbon fibers must be embedded within the copper plate. On theoccasion, local warping generates on the thermal stress relaxation platein high temperature soldering using hard solder. Actually, in theabove-indicated Gazette No. 57-124459, it is described that whensoldering is carried out above 700° C., the warping of the thermalstress relaxation plate suddenly increases.

In addition, since the thermal conductivity and the electricconductivity of carbon fibers are low as compared with those of metal,if large quantity of carbon fibers is embedded in Cu, the total thermalconductivity and the total electric conductivity of the plate are to toolow.

SUMMARY OF THE INVENTION

According to the present invention, a semiconductor device comprises (a)a conductive member having electric conductivity, (b) a first solderlayer provided on the conductive member, (c) a composite plate memberprovided on the first solder layer and comprising (c-1) at least oneframe member made of a first material and having a window space therein,and (c-2) a plate member made of a second material which is insertedinto the window space and is unified with the frame member so that theplate member is substantially in contact with the first solder memberthrough a first opening of the window space, (d) a second solder memberprovided on the composite plate member so that the composite platemember is substantially in contact with the second solder member througha second opening of the window space, and (e) a semiconductor chipprovided on the second solder layer. The first material has acoefficient of thermal expansion equal to or less than three times acoefficient of thermal expansion of the semiconductor chip, and thesecond material is selected from the group consisting of copper, copperalloys, aluminum and aluminum alloys.

The present invention also provides a thermal stress relaxation plateemployable in a semiconductor device for relaxing a thermal stress in asemiconductor chip, comprising (a) at least one frame member made of afirst material and having a window space therein, and (b) a plate membermade of a second material which is inserted into the window space and isunified with the frame member so that the plate member has two surfaceswhich expose at respective window openings of the window space, whereinthe first material has coefficient of thermal expansion equal to or lessthan three times a coefficient of thermal expansion of the semiconductorchip, and the second material is selected from the group consisting ofcopper, copper alloys, aluminum and aluminum alloys.

In an aspect of the present invention, is provided a method of solderinga semiconductor chip on a prescribed position, comprising the steps of(a) preparing a composite plate member comprising at least one framemember made of a first material and having a window space therein, and aplate member made of a second material which is inserted into the windowspace and is unified with the frame member so that the plate memberexposes at first and second openings of the window space, wherein thefirst material has a coefficient of thermal expansion equal to or lessthan three times a coefficient of thermal expansion of the semiconductorchip, and the second material is selected from the group consisting ofcopper, copper alloys, aluminum and aluminum alloys, (b) soldering thecomposite plate member on the prescribed position while making the firstopening face to the prescribed position, and (c) soldering thesemiconductor chip on the composite plate member while making thesemiconductor chip face to the second opening.

According to the semiconductor device of the present invention either ofcopper, aluminum, or their alloys is used as a plate member in thecomposite plate member, so that thermal conductivity and electricconductivity of the composite plate member is high as a whole. Inaddition, since a frame member is made of an alloy having coefficient ofthermal expansion three times as high as or less than that of asemiconductor chip to suppress thermal expansion of a plate member, arelaxation effect of the whole composite plate member against thermalstress of a semiconductor chip is large. As the composite member of thepresent invention can be composed without incorporating such materialsas sintered molybdenum and carbon fibers, they are low in cost andusable even under high soldering temperature conditions. In addition,according to the soldering method of the present invention, asemiconductor device with the above characteristics can be obtainedwithout complicate processing.

Accordingly, an object of the present invention is to provide a low costsemiconductor device equipped with a thermal stress relaxation memberwhich is high in thermal conductivity and electric conductivity and isless in restriction on a soldering temperature.

Another object of the present invention is to provide a method ofsoldering a semiconductor chip for manufacturing such a semiconductordevice.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional view of a semiconductor device accordingto a preferred embodiment of the present invention.

FIG. 2 and FIG. 3 are a plane view and an elevation view of thesemiconductor device in FIG. 1, respectively,

FIG. 4 and FIG. 5 are perspective views of thermal stress relaxationplates with part taken away, and

FIG. 6 is a partial sectional view showing conventional solderingstructure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 and 3 are a plane view and an elevation view showing externalappearance of a semiconductor device 100 according to a preferredembodiment of the present invention, respectively. FIG. 1 is an enlargedpartial sectional view along line I--I in FIG. 2. The semiconductordevice 100 comprises a plurality of power transistor assemblies 20schematically shown in FIG. 3 and a casing 50 receiving these assemblies20. The casing 50 has a heat diffusion plate 60 made of copper as abottom plate on which a resin side member 70 is fixed with an adhesiveagent. A resin lid 80 is fixed on the side member 70 so that it coversupper opening of the side member 70.

On both edges of the casing 50, concaves 51 are formed. As shown in FIG.1, at the bottom of each concave 51, a fitting hole 52 used for fixingthe semiconductor device 100 to a desired position is provided. On theother hand, convexes 81 are provided on three positions of the lid 80. Anut 82 (FIG. 1) is pressed into the convex 81, and a bolt 85 is screwedinto the nut 82 through an external electrode plate 83 and a washer 84.External wires such as bus bars are connected to the external electrodeplate 83 through combination of the bolt 85 and the nut 82. Though notshown in FIG. 1-3, one edge of each external electrode plate 83 is bent,and electrically connected through the lid 80 to an electrode inside thepower transistor assembly 20 in the casing 50.

Fastening terminals 87 are installed on the top of the casing 50, and acontrol signal for the transistor assembly 20 is applied to thefastening terminals 87, and then transmitted to a control electrode inthe assembly 20 through internal wiring not shown. A status in whichpart is removed is depicted in FIG. 1, but in order to protect fromoutside atmosphere the transistor assemblies 20 and each internal wire,the transistor assemblies 20 are sealed with resin 51 inside an internalspace of the casing 50.

Each of the transistor assemblies 20 is manufactured as follows (referto FIG. 1): An insulating base plate 22 made of alumina-ceramics issoldered on the heat diffusion plate 60 through a solder layer 21. Acollector electrode plate 24 made of copper is soldered on the center ofthe upper surface of the insulating base plate 22 through a solder layer23. On the top surface of this collector electrode plate 24, a thermalstress relaxation plate 40 is soldered through a solder layer 25. Thestructure of this thermal stress relaxation plate 40 will be describedlater in detail.

On the thermal stress relaxation plate 40, a semiconductor chip 27 issoldered through a solder layer 26. The semiconductor chip 27 is formedby selectively doping impurities into a silicon substrate and is a powertransisitor containing junctions between a plurality of active regions.

On the top surface of the insulating base plate 22, an emitter electrodeplate 32 made of copper is soldered through a solder layer 31. Likewise,a base electrode plate 35 made of copper is soldered on the plate 22through a solder layer 34. These emitter electrode plate 32 and baseelectrode plate 35 connected to an emitter region and a base region ofthe semiconductor chip 27 through aluminum wires 33 and 36,respectively. The other ones of the transistor assemblies 20 not shownin FIG. 1 also have the same structure, a plurality of the transistorassemblies 20 being connected each other through wiring and their outputbeing led to the external electrode plates 83.

FIG. 4 is a perspective view of the thermal stress relaxation plate 40with part taken away. The thermal stress relaxation plate 40 comprises arectangular frame member 41 and a rectangular plate member 42 insertedinto a window space W₁ defined by the frame member 41. The members 41and 42 are united with each other to form the thermal stress relaxationplate 40. The frame member 41 is made of invar (an alloy consisting ofFe=63.4%, Ni=36%, Mn=0.4% and C=0.2%), and the plate member 42 is madeof cooper.

This thermal stress relaxation plate 40 may be obtained by rolling anassembly in which a copper plate is inserted into a rectangular invarframe. Owing to this rolling, a gap between the frame member 41 and theplate member 42 disappears, so that the members 41 and 42 are unified.

The top and bottom surface of the thermal stress relaxation plate 40 aresubstantially flat, and the plate member 42 is exposed through the upperand lower openings P₁ and P₂ of the window space W₁. In addition, oneach surface of the frame member 41 and the plate member 42, nickelplating is applied for attaining tight soldering of the plate 40.Therefore, in case the thermal stress relaxation plate 40 is mounted asshown in FIG. 1, the plate member 42 is substantially in contact withthe solder layers 25 and 26.

The frame member 41 may be made of covar (an alloy consisting ofNi=23-30%, Co=30-17%, Mn=0.6-0.8% and Fe=balance) in place of invar. Incase the frame member 41 is made of the covar, nickel plating is alsoapplied to the surface. In Table 1, thermal conductivity and coefficientof thermal expansion are shown for invar, covar, copper and silicon,respectively. Besides these values are shown also for molybdenum forcomparison with conventional technology.

                  TABLE 1                                                         ______________________________________                                                              Coefficient of thermal                                         Thermal conductivity                                                                         expansion                                                      (W/mm °C.)                                                                            (10.sup.-6 /°C.)                                 ______________________________________                                        Invar     0.016           5.5                                                 Covar     0.017           5.1                                                 Copper   0.39             17.1                                                Silicon  0.16             3.5                                                 Molybdenum                                                                             0.13             5.0                                                 ______________________________________                                    

As understood from Table 1, the thermal conductivity of copper which isused for the plate member 42 is three times as large as that ofmolybdenum. Besides though not shown in Table 1, copper is extremelyhigh in electric conductivity. Accordingly, the thermal conductivity andthe electric conductivity in an orthogonal-to-plase direction (Zdirection in FIG. 4) through the plate member 42 become extremely highin the thermal stress relaxation plate 40. Therefore, not only heattransmission during soldering is made favorable, but also transientthermal resistance during an electric current flows through thesemiconductor chip becomes less. As a result, the plate 40 can besatisfactorily used for a high speed switching transistor for largepower use.

On the other hand, since the plate member 42 is made of the samematerial, copper, as that of the collector electrode plate 24 and itscoefficient of thermal expansion around five times as high as that ofsilicon, thermal stress due to difference of the coefficient of thermalexpansion between the collector electrode plate 24 and the semiconductorchip 27 cannot be relaxed only by the plate member 42. However, as thecoefficient of thermal expansion of invar or covar, which is thematerial of the frame member 41, is approximately the same as silicon,thermal expansion in intra-plane direction (an X-Y direction) of theplate member 41 can be suppressed by the frame member 41. Moreparticularly, the coefficient of thermal expansion in the X-Y plane isaround 6.0×10⁻⁶ /°C. as a whole in the thermal stress relaxation plate40. Though the thermal expansion in the Z direction of the plate member42 increases owing to the suppression of the thermal expansion in theX-Y plane, overall expansion of the plate member 42 in the Z directiondoes not result in causing stress within the semiconductor chip 27 and,therefore, this phenomenon causes no practical problems.

That is to say, the thermal stress relaxation plate 42 is formed as acomposite plate member consisting of the frame member 41 and the platemember 42, so that the members 41 and 42 meet the first requirement,i.e., improvement of the thermal conductivity and the electricconductivity, and the second requirement, i.e., keeping the coefficientof thermal expansion lower, respectively. In addition, different fromthe conventional technology in which organic materials such as carbonfibers are embedded within a copper plate, the thermal stress relaxationplate 40 is composed as a combination of metals and alloys. Accordingly,the coupling surface between the members 41 and 42 in the thermal stressrelaxation plate 40 is stable and usable at a comparatively hightemperature to allow soldering with hard solder. Besides there is alsono lowering of the electric conductivity due to embedding great quantityof the carbon fibers.

FIG. 5 is a perspective view of a thermal stress relaxation plateaccording to another preferred embodiment of the present invention, inwhich part thereof is taken away. This thermal stress relaxation plate43 is a composite plate member in which rectangular grid-like framemembers 44 and 46 having the same configration each other are mutuallyarranged in parallel, and a plate material 45 is inserted between them.Also in this example, the frame members 44 and 46 are made of invar orcovar, and the plate member 45 is made of copper.

The thermal stress relaxation plate 43 may be obtained from preparing anassembly in which a cooper flat plate is sandwiched between one pair ofgrid-like invar frames or covar frames and rolling this assembly. Eachsurface of the frame members 44 and 46 and the plate member 45 isnickel-plated as that of the thermal stress relaxation plate 40.

The plate member 45 has a center portion 45a extending in an X-Y planedirection and matrix arrangements of convexes 45b and 45c projectingup-and-downward from the portion 45a, respectively. In the matrixarrangement of window spaces W₂ defined by the frame members 44 and 46,the convexes 45b exposes through the bottom openings Q₁ and the otherconvexes 45c expose through the top openings Q₂. Accordingly, in each ofthe window spaces W₂, the plate member 45 extends from the bottomopening Q₁ of the window space W₂ to the top opening Q₂, so that thermalconductivity and electric conductivity in the Z direction are secured bythe plate member 45 as the case of the thermal stress relaxation plate40. In addition, when the thermal stress relaxation plate 43 is used inplace of the thermal stress relaxation plate 40 in FIG. 1, the platemember 45 substantially contacts with the solder layers 25 and 26.

Thermal expansion in the X-Y plane direction within the thermal stressrelaxation plate 43 is limited substantially by the coefficient ofthermal expansion of the frame members 44 and 46. Therefore this thermalstress relaxation plate 43 also possesses favorable properties ofthermal conductivity, electric conductivity and thermal expansion.

Since invar, covar and copper are considerably lower in price comparedto that of molybdenum, the semiconductor device 100 manufactured usingthe thermal stress relaxation plate 40 or 43 is also lower in cost.

In the following, materials usable for a thermal stress relaxation platein the present invention are furthermore discussed. To begin with, asregards to characteristics required for the frame members 41, 44 and 46,it is necessary for their coefficients of thermal expansion to becomparatively near to that of the semiconductor chip 27. The range ofcoefficient of thermal expansion is allowed depends on solderingconditions, but if their coefficient of thermal expansion is equal to orless than three times the coefficient of thermal expansion of thesemiconductor chip 27, a practical thermal stress relaxation effect canbe obtained. In case the semiconductor chip 27 is made of silicon, it isunderstood from Table 1 that an alloy with the coefficient of thermalexpansion equal to or lesss than around 10.5×10⁻⁶ /°C., which is threetimes 3.5×10⁻⁶, may be used as the frame member capable of suppressinginherent expansion of copper. Invar and covar used in the preferredembodiments meet the requirement, as seen from Table 1. Other alloys ofiron satisfying the requirement may be employed as the materials of theframe members 41, 44 and 46.

On the other hand, high thermal conductivity and electric conductivityis required for the plate member 42 and 45. Accordingly, a materialselected among copper, copper alloys, aluminum and aluminum alloys isemployable for plate members 42 and 45. Thermal conductivity of aluminumis 0.24 W/mm°C., being around twice that of molybdenum. On the otherhand, as an example of a usable copper alloy, there is copper containing0.15-0.20% tin, its thermal conductivity being 0.35 W/mm°C. Further, asan example of an aluminum alloy usable for the plate members 42 and 45,there is aluminum containing 1-2% silicon.

The present invention is also appliable for soldering a semiconductorchip made of a semiconductor material other than silicon such as GaAs,and for all semiconductor devices in which semiconductor chips aresoldered to a designated position or member.

As described above, according to the present invention, a compositeplate member composed of a plate member having high thermal conductivityand high electric conductivity and a frame member with coefficient ofthermal expansion comparatively close to a semiconductor chip is used asa thermal stress relaxation member, and these member can be made withoutusing costly materials. Accordingly, a semiconductor device comprising athermal stress relaxation member having high thermal conductivity andhigh electric conductivity can be obtained at lower cost.

In addition, as organic fibers such as carbon fibers are not used, thethermal stress relaxation member according to the present invention canbe used in high temperature soldering and the allowable limit of thesoldering temperature is improved.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation. The spiritand scope of the present invention should be limited only by the termsof the appended claims.

What is claimed is:
 1. A semiconductor device comprising;(a) aconductive member having electric conductivity; (b) a first solder layerprovided on said conductive member; (c) a composite plate memberprovided on said first solder layer and having a first major surfacesubstantially in contact with said first solder member and a secondmajor surface opposite to said first major surface, said composite platemember comprising; (c-1) a closed frame member made of a first materialand serving as a circumferential part of said composite plate member andexposed in said first and second major surfaces of said composite platemember, wherein a space surrounded by said frame member is defined as awindow space; and (c-2) a plate member made of a second material whichis inserted into said window space and is united with said frame memberso that said plate member is exposed in said first and second majorsurfaces of said composite plate member; (d) a second solder memberprovided on said second major surface of said composite plate member sothat said frame member and said plate member are substantially incontact with said second solder member; and (e) a semiconductor chipprovided on said second solder layer; wherein said first material has acoefficient of thermal expansion equal to or less than three times acoefficient of thermal expansion of said semiconductor chip, and saidsecond material is selected from the group consisting of copper, copperalloys, aluminum and aluminum alloys.
 2. The semiconductor device ofclaim 1, wherein;said frame member is a rectangular frame member; andsaid plate member is a rectangular flat plate member inserted into saidwindow space.
 3. The semiconductor device of claim 1, wherein;said firstmaterial is selected from the group consisting of invar and covar. 4.The semiconductor device of claim 1, wherein;said first and second majorsurfaces of said composite plate member are nickel plated.
 5. Thesemiconductor device of claim 1, wherein;said conductive member is anelectrode plate for said semiconductor chip.
 6. The semiconductor deviceof claim 1, further comprising;(f) a heat diffusion plate on which saidconductive member is soldered through an insulating plate.
 7. Thesemiconductor device of claim 6, wherein;said composite plate memberserves as a thermal stress relaxation plate for relaxing a thermalstress in said semiconductor chip.
 8. A semiconductor device,comprising;(a) a conductive member having electric conductivity; (b) afirst solder layer provided on said conductive member; (c) a compositeplate member provided on said first solder layer and having a firstmajor surface substantially in contact with said first solder member anda second major surface opposite to said first major surface, saidcomposite plate member comprising; (c-1) a first lattice member made ofa first material and exposed in said major surface of said compositeplate member, wherein said first lattice member defines a plurality offirst window spaces between respective lattice elements of said firstlattice member; and (c-2) a second lattice member made of said firstmaterial and exposed in said second major surface of said compositeplate member, wherein said second lattice member defines a plurality ofsecond window spaces between respective lattice elements of said secondlattice member and is arranged in parallel with said first latticemember across a gap, respectively; (c-3) a plate member comprising:(c-3a) a center portion inserted into said gap; (c-3b) a plurality offirst convex portions projecting from said center portion into saidplurality of first windows so that said first lattice member and saidplurality of first convex portion are exposed in said first majorsurface of said composite plate member; and (c-3c) a plurality of secondconvex portions projecting from said center portion into said pluralityodf second windows so that said second lattice member and said pluralityof second convex portion are exposed in said second major surface ofsaid composite plate member; (d) a second solder member provided on saidsecond major surface of said composite plate member; and (e) asemiconductor chip provided on said second solder layer; wherein saidfirst material has a coefficient of thermal expansion equal to or lessthan three times a coefficient of thermal expansion of saidsemiconductor chip, and said second material is selected from the groupconsisting of copper, copper alloys, aluminum and aluminum alloys. 9.The semiconductor device of claim 8, wherein;said first and secondlattice members are first and second matrix lattice members,respectively; and respective matrix elements of said first and secondmatrix lattice members are aligned with each other.
 10. Thesemiconductor device of claim 8, wherein;said first material is selectedfrom the group consisting of invar and covar.
 11. The semiconductordevice of claim 10, wherein;said first and second major surfaces of saidcomposite plate member are nickel plated.
 12. The semiconductor deviceof claim 10, wherein;said conductive member is an electrode plate forsaid semiconductor chip.
 13. The semiconductor device of claim 12,further comprising;(f) a heat diffusion plate on which said conductivemember is soldered through an insulating plate.
 14. The semiconductordevice of claim 13, wherein;said composite plate member serves as athermal stress relaxation plate for relaxing a thermal stress in saidsemiconductor chip.
 15. A thermal stress relaxation plate employable ina semiconductor device for relaxing a thermal stress in a semiconductorchip and having first and second major surfaces opposite to each other,said thermal stress relaxation plate comprising:(a) a closed framemember made of a first material and serving as a circumferential part ofsaid thermal stess relaxation plate and exposed in said first and secondmajor surfaces of said thermal stress relaxation plate, wherein a spacesurrounded by said frame member is defined as a window space; and (b) aplate member made of a second material which is inserted into saidwindow space and is united with said annular frame member so that saidplate member is exposed in said first and second major surfaces of saidthermal stress relaxation plate; wherein said first material has acoefficient of thermal expansion equal to or less than three times acoefficient of thermal expansion of said semiconductor chip, and saidsecond material is selected from the group consisting of copper, copperalloys, aluminum and aluminum alloys.
 16. The thermal stress relaxationplate of claim 15, wherein;said first material is selected from thegroup consisting of invar and covar.
 17. The thermal stress relaxationplate of claim 15, herein;nickel plating is applied to said first andsecond major surfaces of said thermal stress relaxation plate.
 18. Athermal stress relaxation plate employable in a semiconductor device forrelaxing a thermal stress in a semiconductor chip and having first andsecond major surfaces opposite to each other, said thermal stressrelaxation plate comprising:(a) a first lattice member made of a firstmaterial and exposed in said first major surface of said composite platemember, wherein said lattice member defines a plurality of first windowspaces between respective lattice elements of said first lattice member;(b) a second lattice member made of said first material and exposed insaid second major surface of said thermal stress relaxation plate,wherein said second lattice member defines a pluraltiy of second windowspaces between respective lattice elements of said second lattice memberand is arranged in parallel with said first lattice member across a gaprespectively; and (c) a plate member comprising; (c-1) a center portioninserted into said gap: (c-2) a plurality of first convex portionsprojecting from said center portion into said plurality of first windowsso that said first lattice member and said plurality of first convexportion are exposed in said first major surface of said thermal stressrelaxation plate; and (c-3) a plurality of second convex portionsprojecting from said center portion into said plurality of secondwindows so that said second lattice member and said plurality of secondconvex portion are exposed in said second major surface of said thermalstress relaxation plate; wherein said first material has a coefficientof thermal expansion equal to or less than three times a coefficient ofthermal expansion of said semiconductor chip, and said second materialis selected from the group consisting of copper, copper alloys, aluminumand aluminum alloys.
 19. The thermal stress relaxation plate of claim18, wherein;said first material is selected from the group consisting ofinvar and covar.
 20. The thermal stress relaxation plate of claim 18,wherein;said first and second major surfaces of said thermal stressrelaxation plate are nickel plated.